Oil country tubular goods (OCTG) which are driven into the ground for excavation of oil wells and natural gas wells can reach an overall length of several thousand meters in some cases. These long oil country tubular goods are constituted by steel tubes having a unit length of ten-some meters which are connected in series by short tubular couplings. The material of steel tubes and couplings is carbon steel, stainless steel, or high alloy steel, depending on the environment of use.
These steel tubes are connected by threaded engagement of a pin having a male (external) threaded portion on its outer peripheral surface and a box having a female (internal) thread on its inner peripheral surface. Typically, a pin is formed on both ends of a steel tube and a box is formed on the interior of a short coupling. A connecting portion constituted by this pin and box is a threaded joint for a steel tube.
A threaded joint for steel tubes requiring a high degree of gas tightness has unthreaded metal-to-metal contact portions which are formed on the end of the male thread of the pin and the base of the female thread of the box. When one end of the steel tube is inserted into the coupling and the male thread of the pin and the female thread of the box are tightened until the unthreaded metal-to-metal contact portions of the pin and the box are made to contact each other with a given amount of interference, a metal-to-metal seal is formed in those portions to provide the joint with an increased gas tightness.
In order to perform periodic inspection and the like, an oil country tubular good is sometimes lifted up and subjected to breakout operation in which the threads of the threaded joint are loosened and each of the steel tubes is disconnected from the coupling. After the completion of inspection or the like of each steel tube, makeup of the oil country tubular good is carried out by again tightening the threaded portions of the tubes and couplings, and the oil country tubular good is reused. The sliding contact surfaces of the threaded portions and the unthreaded metal-to-metal contact portions of the pin and the box are repeatedly subjected to strong friction at the time of makeup and breakout of an oil country tubular good. Accordingly, if a threaded joint does not have sufficient durability with respect to friction, seal defects (poor resistance to leaks) and galling (nonrepairable severe seizing) occur in the threaded portions and particularly in the unthreaded metal-to-metal contact portions of the joint when makeup and breakout are repeatedly carried out using the joint.
Therefore, a threaded joint for oil country tubular goods is (a) of course required to be able to resist tensile forces in the axial direction due to the weight of connected steel tubes, (b) it is required to be able to withstand the pressure of internal and external fluids, and (c) it is required to maintain good leak resistance and galling resistance without the occurrence of seal defects or galling when it is repeatedly used at least four times in the case of casing (large diameter tubes) and at least 10 times in the case of tubing (small diameter tubes). In recent years, there is a trend for oil wells to become deeper and deeper, and the frequency of use in severe environments such as in polar regions is increasing, so the quality required of threaded joints for steel tubes is becoming increasingly strict.
In the past, as proposed in below-described Patent Document 1 and the like, the contact surfaces of the pin and the box of a threaded joint such as the threaded portions of the joint have been treated by surface treatment such as copper plating or phosphate treatment, and for further improvement in galling resistance, the joint interface between the pin and the box is filled with a compound grease called dope which contains heavy metals such as Pb and which is applied each time makeup is performed.
However, under present circumstances in which preventing environmental pollution on a global scale is becoming an urgent problem, the use of dope containing Pb is being regulated. Dope which does not contain heavy metals such as Pb, Zn, and Cu (such dope being called green dope) has been developed and is being used. However, the performance of green dope is not so high, and depending on the material of the threaded joint, it cannot prevent the occurrence of galling.
Other methods of improving the leak resistance and galling resistance of threaded joints for steel tubes which have been proposed in the prior art include (1) a method in which a plating layer containing a powder of a fluororesin dispersed therein is formed, (2) a method in which a lubricating protective film is formed by sputtering, and (3) a method in which a solid lubricating coating is used in place of compound grease, but none of these has achieved sufficient leak resistance and galling resistance.
Below-described Patent Document 2 proposes a threaded joint for oil country tubular goods made of a high Cr steel having an Cr content of at least 9 mass % in which a Cu—Sn alloy plating layer is formed on the female threaded portion and the unthreaded metal-to-metal contact portion (metal-to-metal seal portion) of a coupling is constituting the joint. However, as a result of investigations of this threaded joint by the present inventors, it was found that it had the problem that corrosion easily occurs in the interface between the pin and the box. This corrosion is so-called crevice corrosion, and it becomes marked when green dope is applied as a lubricant at the time of makeup or when a solid lubricating coating or other lubricating coating is formed atop the plating layer. If this corrosion takes place in a threaded joint, leak resistance and galling resistance of the joint decrease due to the rust which develops.
Patent Document 1: JP 01-12995 B1
Patent Document 2: JP 2003-74763 A1